The long-term goal of this project is to identify the neural and neurochemical mechanisms underlying the regulation of maternal behavior and to elucidate potentially novel mechanisms of neural plasticity associated with the expression of maternal behavior in the adult mammal. The specific hypothesis that will be tested is that the induction, maintenance, and retention of maternal care are under endocrine regulation by a neural lactogenic system, a system that displays significant plasticity as a function of reproductive experience. The first series of studies will examine the role of the neural prolactin (PRL) receptor in the initiation of maternal behavior. Using a rat model, we will determine whether placental lactogens, like PRL, act via the PRL receptor to stimulate the onset of behavior. A second study will use the novel PRL receptor antagonist, S179D-PRL, which will be administered centrally to test the hypothesis that activation of the PRL receptor by lactogenic hormones around the time of birth is essential for the normal onset of maternal care. The third study using ISHH will measure how pregnancy concentrations of progesterone (P) and estradiol affect expression of mRNA for neural PRL receptors. Then, central sites of P action will be examined to see how P affects the onset of maternal care and to delineate a mechanism of P's action in the initiation of maternal behavior. A second series of experiments will determine the involvement of the endocrine system in ongoing maternal care. First, the effects of exposure to PRL-secreting ectopic pituitary grafts on pup- directed maternal care and maternal aggression will be measured. Then, the effects of central administration of the PRL receptor antagonist, S179D-PRL, will be examined in lactating rats. The third set of experiments will delineate the role of the endocrine system in the retention of maternal behavior. The involvement of PRL in the opioid-mediated establishment of the retention of maternal behavior will be assessed. Finally, the effects of prior maternal experience on activation of the neural lactogenic system will be measured to see whether prior maternal experience up-regulates the brain PRL system and makes the female more sensitive to her own neural hormones. The results of these investigations will delineate common endocrine and neurochemical regulators of maternal care in mammals, present a new model for examining neuroplasticity in the adult female, and provide a basis for evaluating the effects of endocrine and neurochemical imbalances on mother-young interactions.